1. Field of the Invention
The present invention relates to a method of fabricating a thin film transistor for a liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is fabricated by forming various devices on a transparent insulated substrate, such as a glass substrate. To fabricate a thin film transistor (TFT) on a weak heat-resistant or heat-sensitive substrate, such as a glass substrate, an amorphous silicon layer or a polycrystalline silicon layer is first formed on the substrate. An active layer for the TFT is then formed by etching the silicon layer. Characteristics of charge carrier mobility are excellent when a polycrystalline silicon layer is formed on the glass substrate. Accordingly, when a polycrystalline silicon layer is used, devices for a driver and a pixel array of the LCD device are fabricated on the same substrate simultaneously.
FIG. 1 is a schematic drawing of an LCD device on which a pixel array and a driver circuit are simultaneously formed on the same substrate. The pixel array 11 lies in the middle of the substrate 100, and a gate driver 12 and a data driver 13 are located near two sides of the pixel array 11 on the substrate 100. The pixel array 11 comprises a plurality of pixel cells 14 defined by a plurality of gate lines 15 connected to the gate driver 12 and a plurality of data lines 16 connected to the data driver 13. The gate driver 12 drives the pixel cells. The data driver 13 supplies the cells with data signals. The gate driver 12 and the data driver 13 receive external signals through external signal input terminals 17 and supply them to pixel cells 14. The drivers use inverters of complementary TFTs to generate proper signals for the pixel cells 14.
FIG. 2 is a schematic drawing of silicon grains 21 in a polycrystalline silicon layer which is formed according to a related art. According to FIG. 2, an amorphous silicon layer is formed on a substrate. A laser beam is applied to the amorphous silicon layer and moves by a predetermined distance for each laser pulse. Accordingly, the substrate is scanned by the laser beam, during which the portion of the silicon layer irradiated with the laser beam is melted and crystallized by solidification. By controlling laser energy properly, the portion of the silicon irradiated with the laser beam is almost completely melted, with an unmelted portion remaining at an interface between the silicon layer and the substrate.
In the portion of the silicon layer irradiated with the laser beam, silicon grains grow laterally, rather than in the direction of the thickness of the layer, by using other portions of the silicon that remain unmelted as seeds. Depending on a state of the amorphous silicon layer and the laser energy supply, the seeds are located at an interface between the substrate. Therefore, a polycrystalline silicon layer having grains of irregular sizes at random locations is formed.
When fabricating a TFT of a coplanar type, an active layer is formed by patterning a polycrystalline layer formed on the insulated substrate using, e.g., photolithography. A gate insulating layer and a gate electrode are then formed on the active layer, and a source and a drain region are formed by doping the active layer with impurities to form a TFT for the LCD device.
FIG. 3 shows a schematic layout of a polycrystalline silicon TFT according to the related art. The grain pattern of the active layer has no significance on the channel direction because the silicon grains in the active layer are scattered irregularly on the substrate. Referring to FIG. 3, a plurality of silicon grains 34 are included in an active layer 30 of the TFT. The charges moving from the source region 31 through the channel region 32 toward the drain region 33 of the TFT are greatly affected by the grain boundaries in the channel region. Hence, the charge carrier mobility of the polycrystalline silicon TFT is much smaller than that of a single crystalline silicon TFT. Moreover, physical characteristics of the TFT formed on the substrate are irregular since the polycrystalline silicon layer contains irregular grains.
Accordingly, circuit malfunctioning may occur in the gate and data drivers which include TFTs, causing external signals to be transferred unevenly to the gate and data lines. In addition, the TFTs formed irregularly in the pixel array may cause the image characteristics of the LCD to deteriorate.